$^{229}$Th Nuclear Spectroscopy in an Opaque Material: Laser-Based Conversion Electron M\"ossbauer Spectroscopy of $^{229}$ThO$_2$

TL;DR

This paper demonstrates laser-induced conversion electron Mössbauer spectroscopy of $^{229}$Th in ThO$_2$, enabling nuclear transition probing in opaque materials and paving the way for more compact nuclear clocks.

Contribution

First demonstration of laser-based conversion electron Mössbauer spectroscopy for $^{229}$Th, allowing nuclear studies in opaque materials and potential development of compact nuclear clocks.

Findings

Successfully excited nuclear transition in ThO$_2$ using laser

Detected conversion electrons indicating nuclear decay

Proposed nuclear clock with significantly reduced instability

Abstract

Here, we report the first demonstration of laser-induced conversion electron M\"{o}ssbauer spectroscopy of the $^{229}$Th nuclear isomeric state, which provides the ability to probe the nuclear transition in a material that is opaque to light resonant with the nuclear transition. Specifically, we excite the nuclear transition in a thin ThO$_2$ sample whose band gap ($\sim$ 6 eV) is considerably smaller than the nuclear isomeric state energy (8.4 eV). As a result, the excited nucleus can quickly decay by internal conversion, resulting in the ejection of electrons from the surface. By collecting these conversion electrons, nuclear spectroscopy can be recorded. Unlike fluorescence spectroscopy, this technique is compatible with materials whose work function is less than the nuclear transition energy, opening a wider class of systems to study. Further, because ThO$_2$ can be made from spinless isotopes and the internal conversion decay process reduces the isomeric state lifetime to only $\sim$10 $\mu$s, allowing $\sim$10$^8$ relative reduction in clock interrogation time, a conversion-electron-based nuclear clock could lead to a $\sim$10$^4$ reduction in clock instability.